What is a miRNA and its role?

• A. A microRNA is a small non-coding RNA that base-pairs with target mRNAs to repress translation or promote degradation.
• B. A small coding RNA that makes proteins.
• C. A long noncoding RNA that enhances transcription.
• D. An enzyme that degrades mRNA.

Answer: (A)

miRNAs are small non-coding RNA molecules that regulate gene expression after transcription. They are produced as hairpin precursors and processed into mature ~22-nucleotide sequences, then loaded into a protein complex called RISC with Argonaute. The miRNA serves as a guide, using its seed region to pair with complementary sequences—usually in the 3' untranslated region of target mRNAs. This binding recruits the RISC to the mRNA and typically represses translation and/or promotes mRNA deadenylation and decay, lowering the amount of protein produced. In animals, the pairing is often imperfect, leading mainly to translational repression with eventual degradation; in plants, near-perfect pairing can trigger direct mRNA cleavage. This description matches what miRNAs do and how they function, setting them apart from coding RNAs and from other RNA types. The other options describe different molecules: a coding RNA would make proteins, a long noncoding RNA that enhances transcription is a separate class with distinct roles, and an enzyme that degrades mRNA refers to a degraded-mRNA–processing protein, not a miRNA.